1. Field of Invention
This invention relates to a new and improved torque regulating alternating current induction motor control system.
More particularly, the invention relates to a torque regulating control system for use with variable voltage, variable frequency, induction motors of the type normally used for traction motor drives, and wherein improved control of motor torque is achieved by actually sensing the true motor flux and deriving from the sensed true motor flux an actual torque feedback signal that is employed in regulating the operation of the induction motor.
2. Background Problem
In traction motor drive systems such as are employed in transit cars, electric locomotives and the like, it is desirable to closely control or regulate the actual output torque of variable voltage, variable speed induction motors used in such traction motor drive systems. This requirement is due to the fact that the actual output torque is the primary factor controlling acceleration, motoring speed, deceleration, braking speed, etc., and primarily influences passenger safety and comfort.
There are a number of known mechanical torque regulating methods which employ mechanical means for measuring the actual torque of an induction motor. Such mechanical torque measuring arrangements require physical space for obtaining a mechanical torque measurement and thereafter convert the mechanical measurement to a corresponding electrical signal proportional to actual torque. All of these known mechanical torque measuring arrangements are quite prone to failure, and require extra space in the drive between the motor and the gear unit normally driven by an induction traction motor. Since such space is at a premium and not normally available in high speed transit cars, electric locomotives and the like without considerable sacrifice, the known mechanical torque measuring methods have not been widely used.
Known prior art electrical methods of torque regulation used with variable voltage, variable speed induction motor traction drives have not heretofore employed the true or actual motor torque in an all electrical, closed feedback loop control system. Many attempts have been made to try to regulate torque in an all electrical system in the past; however, all such known systems heretofore have used some component of the motor stator current as a measure of the level of torque that the motor is producing. Such known electrical systems also may have employed the motor slip, the actual motor speed, the actual flux level and, perhaps, the real component of the motor stator current to regulate the motor torque. One such known electrical method of torque regulation is described in U.S. Pat. No. 3,593,083 -- issued July 13, 1971 for an APPARATUS FOR PROVIDING THE PILOT VALUES OF CHARACTERISTICS OF AN ASYNCHRONOUS THREE PHASE MACHINE -- by Felix Blaschke, Inventor, and utilizes a two axes steady state equivalent circuit technique of torque analysis employing sensed stator line current as the measure of torque. This known system is sensitive to changes to the internal parameters of the motor due to changes in temperature, and can result in an error in the actual torque calculation by as much as 25% or more.
Difficulty is encountered with previously known electrical methods of motor torque regulation which sensed and utilized the stator current as a measure of the motor torque due to the fact that the resultant torque feedback control signal derived from such system not only is sensitive to the power converter wave form, but is quite susceptible to losses in the motor due to stator resistance changes, changes in the physical parameters of the rotor due to heat, harmonic losses or harmonics occurring in the line voltage supplying the motor. Consequently, such known electrical methods of torque measurement and regulation have not been entirely satisfactory.